Hi everyone, I'm having ongoing issues with engine performance on a new-to-me 92 Sunbird w/ a 1996 rebuild shortblock. OMC V6 4.3, electric fuel pump, Holley 2 bbl.
Here's the problem that I've been chasing for a few weeks: Boat runs fantastic for the first 5 minutes or so. Gets to WOT RPM no issues. Begins to have trouble maintaining high engine speeds after 5-10 min, then starts to fall off. Over the course of a warm day, with a few rests of 30 min-1 hr, the performance falls off over the day and during running. 4k RPM tops, then 3.5k, then hard to maintain 3k. After an hour of pulling a tube, it can't get on plane. Park it for an hour and the performance will come back a bit but not like the beginning of the day. It always starts and idles flawlessly, and it has always been able to maintain 2-2.5k RPM sounding very healthy even during the worst of the problems.
Fuel delivery was the obvious first place to look, but after several weeks of cleaning, inspecting, and testing, I am pretty sure I have no problems "boat side" with fuel or delivery. Here's what I have done:
- Fuel pump is always putting out 7-8 psi even during "bogging". I have a fuel pressure gauge plumbed in and ziptied up on the dash. I think this takes the pump out of consideration... is there any way that the pump could be at fault if it's supplying good pressure at all times? I have also run with a clear tube in place and there are no air bubbles.
- I have inspected the anti-siphon valve, hoses, pickup, and pickup screen. Everything looks great. Vent appears to be fine from blowing into it.
- Fresh gas, fully filled.
- Each time I run it hard I dump the water separator contents into a clear jar. No water to speak of and the gas looks great.
- Today I ran a separate fuel tank on the deck with a clear hose today as a test for a while and the problem happens on a separate fuel tank. So I think I have completely ruled out fuel delivery.
Carb?
- I completely rebuilt the carb and corrected many problems, most related to rich running. Really cleaned up the plugs and transom but did nothing to the bogging issue.
- Float height is perfect, newer float needle (2009), no sticking (on the bench), very clean float chamber.
- Inside of the carb is completely clean and has fresh gaskets, clean inlet filter. I used a Holley kit and the parts are really nice (better than the junk that was used during the last rebuild.) Bogging is same before and after rebuild.
- All passages are clear and have been liberally cleaned and blown out with air
- Since the boat runs like a bat out of hell early in the day I'm not going to spend much more time on the carb
Some other things:
- Compression is great. Motor is really healthy. Oil is clean and proper level
- flame arrestor is clean
- Not overheating, steady 160F temps
- Timing on the muffs is spot on, 8 BTDC and advances to 18+
- Wires look fresh, cap, rotor, and plugs are newer (2009)
- I have not torn into the ignition system other than looking at the plugs and setting timing.
- FWIW idle manifold vacuum is good at about 16 in-hg. Needle is pretty steady.
Ideas
- So now I'm trying to think of anything heat-soak related which can deteriorate over the day, and for now not considering any other fuel delivery issues.
- Based on the strong pressure and no bubbles I haven't checked pump vacuum (pressure is great always), but I can't wrap my brain around any way it could be vacuum related
- From the Adults Only WOT section, it mentions "Restricted exhaust system" ... I haven't had this apart so I don't know how it works ... could there be something in there that is heat/time related?
- Sticking float when hot? It looks perfect on the bench...
- Ignition ... my next mission. Bad coil? Cracked distributor? Sticking advance? Do I need to check dynamic timing when at wot?
That's all I can come up with for now. Any ideas about what I have missed before I move onto ignition?
I'll try to put up a picture of my fuel pressure/fuel pump test rig on the dash. I get a lot of laughs about it
Also I want to take a picture of my plugs and see if there are any spark plug whisperers out there who can tell me what the color and markings mean.
Thanks iboats forums!
Here's the problem that I've been chasing for a few weeks: Boat runs fantastic for the first 5 minutes or so. Gets to WOT RPM no issues. Begins to have trouble maintaining high engine speeds after 5-10 min, then starts to fall off. Over the course of a warm day, with a few rests of 30 min-1 hr, the performance falls off over the day and during running. 4k RPM tops, then 3.5k, then hard to maintain 3k. After an hour of pulling a tube, it can't get on plane. Park it for an hour and the performance will come back a bit but not like the beginning of the day. It always starts and idles flawlessly, and it has always been able to maintain 2-2.5k RPM sounding very healthy even during the worst of the problems.
Fuel delivery was the obvious first place to look, but after several weeks of cleaning, inspecting, and testing, I am pretty sure I have no problems "boat side" with fuel or delivery. Here's what I have done:
- Fuel pump is always putting out 7-8 psi even during "bogging". I have a fuel pressure gauge plumbed in and ziptied up on the dash. I think this takes the pump out of consideration... is there any way that the pump could be at fault if it's supplying good pressure at all times? I have also run with a clear tube in place and there are no air bubbles.
- I have inspected the anti-siphon valve, hoses, pickup, and pickup screen. Everything looks great. Vent appears to be fine from blowing into it.
- Fresh gas, fully filled.
- Each time I run it hard I dump the water separator contents into a clear jar. No water to speak of and the gas looks great.
- Today I ran a separate fuel tank on the deck with a clear hose today as a test for a while and the problem happens on a separate fuel tank. So I think I have completely ruled out fuel delivery.
Carb?
- I completely rebuilt the carb and corrected many problems, most related to rich running. Really cleaned up the plugs and transom but did nothing to the bogging issue.
- Float height is perfect, newer float needle (2009), no sticking (on the bench), very clean float chamber.
- Inside of the carb is completely clean and has fresh gaskets, clean inlet filter. I used a Holley kit and the parts are really nice (better than the junk that was used during the last rebuild.) Bogging is same before and after rebuild.
- All passages are clear and have been liberally cleaned and blown out with air
- Since the boat runs like a bat out of hell early in the day I'm not going to spend much more time on the carb
Some other things:
- Compression is great. Motor is really healthy. Oil is clean and proper level
- flame arrestor is clean
- Not overheating, steady 160F temps
- Timing on the muffs is spot on, 8 BTDC and advances to 18+
- Wires look fresh, cap, rotor, and plugs are newer (2009)
- I have not torn into the ignition system other than looking at the plugs and setting timing.
- FWIW idle manifold vacuum is good at about 16 in-hg. Needle is pretty steady.
Ideas
- So now I'm trying to think of anything heat-soak related which can deteriorate over the day, and for now not considering any other fuel delivery issues.
- Based on the strong pressure and no bubbles I haven't checked pump vacuum (pressure is great always), but I can't wrap my brain around any way it could be vacuum related
- From the Adults Only WOT section, it mentions "Restricted exhaust system" ... I haven't had this apart so I don't know how it works ... could there be something in there that is heat/time related?
- Sticking float when hot? It looks perfect on the bench...
- Ignition ... my next mission. Bad coil? Cracked distributor? Sticking advance? Do I need to check dynamic timing when at wot?
That's all I can come up with for now. Any ideas about what I have missed before I move onto ignition?
I'll try to put up a picture of my fuel pressure/fuel pump test rig on the dash. I get a lot of laughs about it
Also I want to take a picture of my plugs and see if there are any spark plug whisperers out there who can tell me what the color and markings mean.Thanks iboats forums!
